The region occupied by San Diego Bay and the mesa back of it is composed, as far as we know, of Quaternary, Pliocene, and perhaps Miocene strata. Sandstones characterize the lower formation, and loosely cemented conglomerates, increasing in thickness toward the mountains, the upper. These were deposited in a sort of basin, of which Point Loma and the Soledad hills formed the northern and western borders. Many oscillations of level have taken place, the most recent being an elevation of 40 feet, shown by an old beach line on Point Loma. The shells in this beach are the same as those now living in the adjoining ocean. It is a peculiar fact that the mesas are slightly higher near their western terminations than farther east, indicating a recent uplift along the ocean. Water is scarce through this mesa formation. At the end of Point Loma there is a strong sulphur spring exposed at low tide. Its waters may possess medicinal properties, and should be examined.
On the southern shore of False Bay is a large deposit of calcareous tufa. The central portion is quite pure and a number of feet thick; just how thick is not known. It extends along the shore some distance, and often contains bowlders and shells. This is evidently a deposit from some former spring of great size. The mesas lying west of the extensive volcanic tuffs have been derived largely from the decay of the latter, and have heavier soil. North of Soledad they become more sandy, and maintain this character to the Santa Margarita Creek. This light soil, however, is being successfully cultivated in many places and for certain kinds of fruit, without irrigation. The surface of the higher portion of Point Loma, as well as some of the mesas north, is covered with spherical nodules, a quarter to half an inch in diameter, of sand cemented with red oxide of iron. These literally cover the ground in places so that it is difficult to walk. The origin of these at first seemed very puzzling, but on examining the face of a cliff on the top of which these were found, an explanation was reached. They were seen to grow smaller away from the surface of the ground until a depth of 2 feet was reached, when they cease. Their formation is due to the oxidation of the iron in the sandstone, and its segregation in little nodules on the same principle as the formation of concretions.
Fig. 13.
The cliffs of Eocene sandstone along the ocean grow gradually lower north of Soledad Cañon. At Encinitos the cliffs are higher again and for a short distance the strata dip south, but toward Oceanside they resume the northerly dip and disappear several miles south of that place. Faults grow less numerous the farther we get from Point Loma. The mesa is low about Oceanside; it was either never very prominent or else the erosion has been great.
On the north bank of the Santa Margarita Creek, near the ranch house, is an interesting cliff of Quaternary sands and gravel, showing a number of strata deposited under different conditions on an old beach. ([Fig. 13].)
The Tertiary beds north of the Santa Margarita Creek are very different in outline from those south. Instead of their extending in a gradual slope from the older mountains to the ocean, there arises in them, near their western border, a range of mountains, known as the San Onofre Mountains. These extend parallel to the ocean at an average distance of 2 miles. They rise north of the Santa Margarita Creek and extend to the San Onofre Creek. They have a gradual slope on the west, rising to an elevation of 1,400 feet, but are quite abrupt on the east. Los Flores Creek cuts through the southern end of this range, showing that while the soft, clayey sandstones between it and the Santa Margarita Mountains slope only 5° to 10° southwest, the rocks of the range itself dip west at an angle of 35° to 40°. The formation is a breccia, the fragments of which are argillitic, micaceous, and hornblendic schists. Some of these fragments are of great size, one bowlder of hornblende schist being 8 feet in diameter. Pebbles of white quartz and other hard metamorphics are also present. The soft, coarse sandstone in which the fragments are imbedded show no traces of any granitic matter. The range was ascended 2 miles north of the Los Flores ranch house, and found to consist entirely of fragmental schists, such as those mentioned, dipping southwest at an angle of 45°. The mountains were also climbed at their northern end, near San Onofre Creek. Here there is a very abrupt escarpment on the eastern side. The strata dip toward the ocean at a high angle, while the irregular hills and ridges of soft, light-colored sandstone lying east toward the Santa Margarita Mountains are nearly level. After a careful study of the range the conclusion was reached that its origin was due to a great fault, represented by the very abrupt eastern slope, tilting the elevated portion to the west at a high angle. I believe that this fault took place after the deposition of the Tertiary strata. As far as my observation went the Tertiary beds on the east do not rise to meet the San Onofre range, as they would to a certain extent if it were present when they were deposited; on the contrary, they dip toward it. West of the range the ocean is bordered by very high cliffs of Quaternary clays, and in only two or three places do the Tertiary rocks outcrop. Small patches of sandstone outcrop near the road at the western foot of the mountains; they also dip west at a high angle. Many of the fragments at the northern end of the range show their derivation from a massive crystalline rock. The hornblende schists are generally garnetiferous. Blue glaucophane schists are also very common. South of Mission Viejo Creek, Orange County, there is an outcrop of rock, apparently in place, which greatly resembles these schists. Good outlines of these mountains, indicative of structure, can be seen to great advantage from the San Luis Rey Mission. On the west slope of the San Onofre Mountains, 4 miles north of Los Flores, is an outcrop of a garnetiferous hornblende schist, which certainly appears to be in place. This rises 10 feet above the side of a gulch, and is fully 20 feet across. One mile north and in line of strike with the last is another outcrop of similar rock, which is so large that it certainly seems that it must be in place. The only point north of the San Onofre where this breccia appears is at Arch Beach, Orange County. The Santa Margarita Mountains are bordered by very extensive bowlder deposits, which rise as high as 1,500 feet on their western side.
The topography of the northwestern part of the county between Temecula, Elsinore, and the ocean, is very complicated. This section is occupied by rugged, brush-covered mountains and narrow, deep valleys, with the exception of the Santa Rosa plateau, where the configuration of the county has been entirely changed by extensive lava eruptions, stretching over a distance of 10 miles. This mountain region narrows toward the north to form the Santa Ana range. The variety of rock formations is very large. The northern portion is unsurveyed. On the south are the two large grants, the Santa Margarita and the Santa Rosa. Between these lies De Luz Valley. The Santa Margarita Mountains extend north and south, forming the eastern borders of the grant and rising to an elevation of 3,100 feet. The granite of the region about De Luz Valley is far from being homogeneous. A part of it is undoubtedly intrusive, and a part may represent an original sedimentary formation. Bedding planes are present in much of this supposed metamorphic granite, but generally no schistose structure. The presence of the De Luz warm springs is perhaps due to a dike of dark, aphanitic diorite, which has cut through the granite in an irregular manner. A very interesting breccia outcrops in the bed of the creek below the warm springs. ([Fig. 14].)
Fig. 14.